This invention relates to digital computer network technology. More specifically, it relates to methods and apparatus for improving packet performance on upstream and downstream channels of an access network.
Broadband access technologies such as cable, fiber optic, and wireless have made rapid progress in recent years. Recently there has been a convergence of voice and data networks which is due in part to US deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice, video, and other high-bandwidth applications over their local access networks. For networks that use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), providing reliable high-quality voice/video communication over such networks is not an easy task.
A cable modem network or “cable plant” employs cable modems, which are an improvement over conventional PC data modems and provide high speed connectivity. Cable modems are therefore instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services. Digital data on upstream and downstream channels of the cable network is carried over radio frequency (“RF”) carrier signals. Cable modems convert digital data to a modulated RF signal for upstream transmission and convert downstream RF signal to digital form. The conversion is done at a subscriber's home. At a cable modem termination system (“CMTS”) located at a Head End of the cable network, the conversions are reversed. The CMTS converts downstream digital data to a modulated RF signal, which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the digital data is fed to the cable modem (from an associated PC for example), which converts it to a modulated RF signal. Once the CMTS receives the upstream RF signal, it demodulates it and transmits the digital data to an external source.
FIG. 1 is a block diagram of a typical two-way hybrid fiber-coaxial (HFC) cable network system. It shows a Head End 102 (essentially a distribution hub) which can typically service about 40,000 homes. Head End 102 contains a CMTS 104 that is needed when transmitting and receiving data using cable modems. Primary functions of the CMTS include (1) receiving baseband data inputs from external sources 100 and converting the data for transmission over the cable plant (e.g., converting Ethernet or ATM baseband data to data suitable for transmission over the cable system); (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable system.
Head End 102 connects through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each Head End can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the Head End and each distribution node. In addition, because cable modems were not used, the Head End of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112, which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (different frequency ranges are used for upstream and downstream paths). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along with trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
In order for data to be able to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is DOCSIS. The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.) in document control number SP-RFIv1.1-I03-991105, Nov. 5, 1999. That document is incorporated herein by reference for all purposes.
Data Communication in Cable Networks
According to the DOCSIS standard, data is transferred by a request-grant mechanism. In order for a cable modem to transmit data to the Head End, it must first send a data grant request to the CMTS. The CMTS, on receiving a request from a cable modem on a particular upstream channel, allocates one or more time slots on that upstream channel for the cable modem to send its data.
The CMTS includes a Media Access Control (MAC) scheduler which is responsible for scheduling types of slot allocations for one or more upstream channels. According to the DOCSIS standard, the scheduling for the shared use of a particular upstream channel is coordinated using channel MAP messages generated specifically for that upstream channel. The CMTS includes a MAP generating device which communicates with the MAC scheduler for generating MAP messages for one or more designated upstream channels. The channel MAP messages which are generated for a particular upstream channel are broadcast to each of the cable modems using that particular upstream channel to communicate with the CMTS. Each MAP message may include a plurality of different time slot types allocated for different purposes, such as, for example, data grant slots, initial ranging slots, maintenance slots, etc.
When the CMTS receives a data grant request from a cable modem on a particular upstream channel, the CMTS schedules a future data grant slot allocation in a MAP message to be broadcast (on a downstream channel) to the requesting cable modem. When the MAP message is received at the cable modem, the cable modem identifies the data grant slot which has been allocated to it, and broadcast its data on the upstream channel at the specific time corresponding to the allocated data grant slot.
As commonly known to one having skill in the relevant art, upstream packet performance in a cable network may be effected by a variety of different factors. One factor which effects the upstream packet performance relates to the delays associated with a cable network. Performance of data transmission and other aspects of the cable network are influenced by different types of delays which are inherent in the cable network. For example, when the CMTS receives a data grant request from a specific cable modem on a particular upstream channel, the time value of the data grant slot allocated to this cable modem must be far enough ahead in the future to account for the delays associated with, for example, MAP construction, propagation of the MAP message to the cable modem, processing of the MAP message by the cable modem, etc. Moreover, since many of the delay values inherent in the network vary depending upon specific network conditions, conventional cable networks are typically configured to operate using maximized delay values based upon anticipated worst-case conditions in the network. As a result, optimal performance of data transmission across the network is compromised. Further, as the use of cable networks or other access networks proliferate in the marketplace, conventional techniques for implementing data communication over a cable network may be not be sufficient for handling larger volumes of traffic which may be caused, for example, by a greater number of users accessing the Head End or by new and emerging broad band network applications such as, for example, telephony.
Accordingly, there exists a continual need to improve data communication performance over an access network in order to accommodate heavier traffic patterns which may be due to new and emerging network applications and technologies, as well as a greater demand for broadband access to shared to networks such as the internet.